1. Field of the Invention
The present invention relates to a pressure transducer, and more particularly to a pressure transducer having error compensation.
2. Discussion of the Related Art
Pressure transducers using strain gauges in a Wheatstone bridge configuration are wellknown in the art. Such pressure transducers are sensitive to various disturbances, such as temperature changes, which, if uncompensated, will cause errors in the pressure reading. Temperature induced errors may be observed as a change in the output of the transducer as temperature varies with zero pressure applied, and as a change in the difference between the full-scale output and the zero pressure output as the temperature varies with full-scale pressure applied. These errors are known as "thermal effect on zero (or offset)" and "thermal effect on span", respectively.
Methods are well-known in the art to compensate for such errors and initially require characterization of the transducer to define any errors. Typically, at least two points from the output signal of the transducer are recorded as temperature is varied over a desired range both with zero pressure applied and with some amount of pressure applied. The pressure applied is typically, but not necessarily, full-scale pressure, and the output is recorded at the same temperature points with zero pressure and with the applied pressure. Based on the output signals, the uncompensated thermal effects are calculated and used to derive the required amount of compensation.
Any of several methods can be used to provide error compensation in a pressure transducer. One common method is to add resistors in series with the bridge supply voltage, and in series with and/or in parallel to the individual bridge resistors. The resistors are chosen based on the particular thermal properties necessary to negate the observed thermal effects, and their values are calculated based on the uncompensated thermal measurements. Error compensation may also be accomplished by laser trimming resistors or thermistors to force voltage changes at the sensor. Another method, known as digital compensation, uses stored data to generate error-correction signals which are added to or subtracted from the uncompensated output of the bridge.
Error compensation to achieve accurate pressure measurements, however, can be a costly and time-consuming process. Frequently, the process of characterizing the transducer, adding compensation, re-characterizing and adjusting the compensation must be repeated several times to obtain the desired accuracy. This can be more difficult with particular transducer designs; for example, in micro-machined silicon sensors with full-scale pressures of 1 inch (1") H.sub.2 O or less.
Acceleration and gravity are additional factors that can affect the sensitivity of pressure transducers, particularly for use in low pressure applications due to the relatively high mass of their diaphragms in relation to the small force necessary to deflect them. While acceleration forces may not be a factor in all applications, gravity is omnipresent and can cause transducers to be sensitive to their mounting position. Error compensation for acceleration and gravity typically requires using complex structures that are expensive and difficult to make.
Warm-up errors and drift are also factors that affect the sensitivity of even a well-compensated transducer. Warm-up errors and drift occur when a transducer is first turned on due to a thermal lag between components. This cannot generally be reduced by existing compensation methods, but requires highly stable or closely matched components that can substantially increase the product cost.
It is also known to make a pressure transducer using a thin, silicon chip on which have been formed a number of resistances that function as strain gauges. As the cost of these silicon strain gauges has decreased, it has been suggested to interconnect two of these silicon transistors so that errors associated with one sensor cancel the errors in the other sensor. In particular, U.S. Pat. No. 4,565,097 discloses a pair of interconnected wheatstone bridge sensors. In the '097 patent, the resistances of one sensor are connected in the same portion of the bridge with an opposing element of the other sensor so that offsets and drifts are opposed by and largely cancelled by those of the other sensor. Since the pressure of interest is applied to only one of the sensors in the pair of sensors, however, the pressure transducer produces an output that is not cancelled by the other sensor.
Although the pressure transducer illustrated in the '097 patent may result in the cancellation of temperature effects, drifts, and offsets, it still requires that the errors of each of the sensors that make up the pressure transducer be characterized so that sensors having opposite error effects are paired together. For example, if the first sensor in the pressure transducer has a positive temperature coefficient, the second sensor to be used for cancellation of the positive temperature coefficient should have a negative coefficient so that when the sensors are connected together to form the pressure transducer, the positive and negative temperature coefficients will cancel each other out.
In addition, the '097 patent wires the two sensors together so that the resistors that make up each leg of the wheatstone bridge are placed in series with each other. Due to this series connection of the resistances, the '097 patent requires that the connections between the resistances of the bridge be externally accessible. This can be somewhat inconvenient when working with silicon strain gauges that have already been completely constructed in a particular die. Additionally, the series connection of resistances in the '097 patent requires that the two sensors to be paired together have opposite error characteristics so that the errors will cancel when the bridges are wired together.